1. Field of the Invention
The present invention relates to wireless communication, and more particularly, to a method and apparatus of providing a reference signal used for uplink transmission.
2. Related Art
Wireless communication systems providing a variety of kinds of communication services such as audio and data are widely developed. In general, a wireless communication system is a multiple access system capable of supporting communication with multiple users by using available system resources (bandwidths, transmission power, etc.). Examples of the multiple access system include CDMA (Code Division Multiple Access) system, FDMA (Frequency Division Multiple Access) system, TDMA (Time Division Multiple Access) system, OFDMA (Orthogonal Frequency Division Multiple Access) system, SC-FDMA (Single Carrier-Frequency Division Multiple Access) system, etc.
An MIMO (Multiple Input Multiple Output) system employs Nt (Nt≧1) transmit antennas and Nr (Nr≧1) receive antennas. An MIMO channel composed of the Nt transmit antennas and the Nr receive antennas may be decomposed into Ns independent channels. Here, Ns≦min{Nt, Nr}. An independent channels may be referred to as a spatial layer. A rank has a value corresponding to the number of available independent channels. The MIMO system can provide high spectral efficiency, improved throughput and high reliability through multiple independent channels.
The MIMO system can be classified into SU-MIMO (Single User MIMO) and MU-MIMO (Multi-User MIMO). The SU-MIMO is used to increase a peak data rate for each user while the MU-MIMO is used to increase call (or sector) capacity. The SU-MIMO uses spatial multiplexing and/or spatial diversity in order to provide high throughput and reliability. The MU-MIMO uses SDMA (Spatial Division Multiple Access) or multi-user multiplexing in order to increase capacity.
SC-FDMA has a peak-to-average power ratio (PAPR) lower than that of OFDMA because of single carrier property while having complexity similar to that of OFDMA. Low PAPR is advantageous to user equipment (UE) in terms of transmission power efficiency, and thus SC-FDMA is adopted for uplink transmission in 3rd generation partnership project long term evolution (3GPP LTE) as described in the Clause 5 of 3GPP TS 36.211 V8.2.0 (2008-03) “Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation (Release 8)”.
The uplink transmission in the 3GPP standard defines only single stream transmission trough a single transmit antenna. Even though a UE has two or more transmit antennas, only transmit antenna selection or transmit antenna switching is supported and multi-stream transmission or precoding is not supported.
In general, data can suffer from easily through a wireless channel (so called frequency selective channel). So a reference signal (RS) that already known to both a transmitter and a receiver is transmitted with data information at the same time for channel estimation. The RS may be referred to as a pilot. There are two kinds of roles which the RS does: demodulation and channel measurement. The RS for demodulation may be called as demodulation RS (DMRS). DMRS can be classified into a precoded RS and a non-precoded RS whether to be multiplexed with precoding matrix or not.
In the Clause 5.5 of the 3GPP TS 36.211, at least one resource block is allocated to a PUSCH (Physical Uplink Shared Channel) that carries uplink data and a RS having a length corresponding to the allocated resource block is assigned. A resource block represents time/frequency resources. A resource block is defined as 12 subcarriers over a slot. Multiple UEs may simultaneously transmit data and RSs on the same resource block. The sequences used for the RSs may be defined by cyclic shifts of a base sequence. The base sequence may be Zadoff-Chu (ZC) sequence.
Meanwhile, ITU (International Telecommunication Union) is standardizing IMT-Advance system which provides 1 Gbps at a low rate and 100 Mbps at a high rate to support IP (Internet Protocol) based multimedia seamless service as a next-generation mobile communication system following 3rd generation mobile communication systems. The 3GPP considers a 3GPP LTE-Advanced (LTE-A) system as a candidate technology for the IMT-Advanced system. The LTE-A system is developed to achieve an improved LTE system and to maintain backward compatibility with the LTE system.
Among requirements of the LTE-A system, the improvement of an uplink peak data rate is particularly emphasized because only SC-FDMA using a single antenna is difficult to satisfy peak data rate requirement required by the IMT-Advanced system. An MIMO system that supports multi-stream transmission or precoding and a hybrid system of OFDMA having high frequency efficiency and the existing SC-FDMA are considered as a system for improving a data rate.
One of important issues for data transmission is the design of RS. In the MIMO system or hybrid system for uplink transmission, RS design, RS allocation and related signaling are not defined yet.